Britain says its troops are making a major push into Iraq's second city, Basra. The offensive comes as the British defense secretary warns of potential stiff resistance in Baghdad.

On Sunday some of the British tanks that have been parked outside of Basra for two weeks rolled into the center of the city of more than a million residents.

Defense Secretary Geoff Hoon told British television the offensive is attacking militiamen loyal to Iraqi President Saddam Hussein.

"There was an important operation overnight," said Mr. Hoon, "and we are continuing to put a great deal of pressure on the militias in Basra who are both continuing to intimidate the local population there as well as offering some resistance to our forces."

Mr. Hoon said American troops deployed around Baghdad will probably have to battle with militias for control of the capital. "Baghdad may prove to offer the same kind of problem that Basra has. That is, some resistance from the militias but also a need to avoid significant civilian casualties," the British official continued. "I'm sure the Americans will approach this in a cautious way. But it is right that I should pay tribute to the remarkable progress they have made to date."

Mr. Hoon would not put a time frame on how long British troops will remain in Iraq, but he stressed they will stay on at least for an initial period after the shooting stops to begin repairing the war damage.

"It is important that British forces, coalition forces, do not stay in Iraq a day longer than necessary and certainly we want to see British forces coming home as soon as possible," said Mr. Hoon. "But when I announced the forces for Iraq, for this operation, I made clear that this was a flexible force, that they would be there both for war fighting and indeed for an initial period of rebuilding and reconstruction."

Mr. Hoon said a second British relief ship, the Sir Percival, plans to dock in the Iraqi port of Umm Qasr on Monday to deliver more humanitarian supplies.